Wasps lock Simon Shaw is flying to Australia, after X-rays confirmed that Danny Grewcock has broken his hand.
Bath captain Grewcock suffered a blow to his left hand during England's victory over Uruguay on Sunday and has been ruled out of the rest of the tournament.
"Danny has broken his hand and Simon Shaw is on the way now," said England head coach Clive Woodward.
Shaw was summoned on Sunday as the England medical team feared the worst for Grewcock and they confirmed the switch on Monday.
Grewcock's latest injury is his second since England arrived in Australia.
He suffered a broken toe when a team-mate accidentally trod on his foot during the pre-match warm-up before the Pool C opener against Georgia in Perth last month.
The Uruguay game was Grewcock's first appearance in the tournament after recovering from that setback.
The 30-year-old Shaw missed out behind Grewcock, Martin Johnson, Ben Kay and Martin Corry in Woodward's original squad.
At the time, Woodward described his omission as one of the toughest selection calls he has had to make during his six years in charge of England.